1. Field of the Invention
The field of the invention is fault detection circuits for computer systems. More specifically the field of the invention is those systems which detect errors in the transmission of digital data within computer systems such as programmable controllers.
2. Brief Description of the Prior Art
Fault detection is a desirable feature for most computing systems. Undetected errors lead to incorrect or unintelligible results. In programmable controller computing systems, the transmission of incorrect data to the output circuits causes wrong functions to be performed, potentially resulting in damage to equipment controlled by the programmable controller.
Most larger computer systems therefore use a system to check for errors in the transmission of data within the system. Virtually all large computers use a parity check system to detect faults. In such systems one (or more) "parity" bits of a multi-bit data group (byte) contain information with respect to the contents of the other bits within the group. For example, one bit may indicate whether the sum of the other bits is odd or even. After the digital signal has been transmitted the sum of the other bits is recalculated and compared with the parity bit received. Proper oddness or evenness of the sum in comparison with the parity bit is indicative of a correct transmission.
The need for detecting data transmission errors in small computers and microprocessors (including programmable controllers) is also important. However, parity check systems are relatively much more burdensome to programmable controllers which often use single bit data transmission. Some programmable controllers are known which use four or eight bit signals. The use of a parity bit system in small computing circuits tends to increase expense, reduce the capacity of the circuits, and slow down the operation of the circuits. Therefore, some small computers, microprocessors and many programmable controller systems do not use any fault detection system. Other systems do use some form of fault detection at the sacrifice of the simplicity and efficiency of the system. U.S. Pat. No. 4,118,792 to Struger discloses an example of one such programmable controller which uses a complex system for the detection of errors in data transmission.